1. Field of the Invention
The present invention relates to an LED drive circuit and an LED illumination unit.
2. Description of the Related Art
LEDs have features such as a low current consumption and a long life; they are used not only for display devices but also for illumination devices and the like, and the number of the applications of LEDs is being increased. In an LED illumination device, in order to obtain a desired illumination, a plurality of LEDs are often used (for example, see JP-A-2006-319172).
A general illumination device often uses an AC 100V commercial power supply; with consideration given to a case, for example, where an LED illumination unit is used instead of a general illumination unit such as an incandescent bulb, as with the general illumination unit, the LED illumination unit preferably uses an AC 100V commercial power supply.
When light control is performed on an incandescent bulb, a phase control dimmer (generally called incandescent light control) is used in which a switching element (in general, a thyristor element or a triac element) is turned on at a certain phase angle of an alternating-current power supply voltage, and thus it is possible to easily perform, with a single volume element alone, light control on power supply to the incandescent bulb (for example, see JP-A-2005-26142). It is known that, even when a phase control dimmer performs light control on an incandescent bulb, an incandescent bulb whose wattage is low is connected to a dimmer, and thus flickering or flashing occurs, with the result that the light control cannot be properly performed.
When light control is performed on an LED illumination unit using an alternating-current power supply, as in the case where light control is performed on an incandescent bulb, it is preferable to use a phase control dimmer. Here, a conventional example of an LED illumination system that can perform light control on an LED illumination unit using an alternating-current power supply is shown in FIG. 7.
The conventional LED illumination system shown in FIG. 7 includes a phase control dimmer 200, an LED drive circuit 300 and an LED load 400 composed of a plurality of LEDs. The LED drive circuit 300 includes a full-wave rectifier 1 and an LED current control circuit 2. The phase control dimmer 200 is connected in series between an alternating-current power supply 100 and the LED current control circuit 2. When the control (not shown) of a half fixed resistor Rvar is set at a certain position, the phase control dimmer 200 turns on a triac Tri at a power supply phase angle corresponding to the set position. Furthermore, in the phase control dimmer 200, a noise reduction circuit formed with a capacitor CLC and a coil LLC is provided, and thus a terminal noise that is fed back from the phase control dimmer 200 to a power supply line is reduced.
A conventional incandescent bulb illumination system that performs, with a phase control dimmer, light control on an incandescent bulb is shown in FIG. 8. Voltage and current waveforms of portions of the conventional incandescent bulb illumination system shown in FIG. 8 are shown in FIG. 9. When a triac Tri is turned on, a voltage V3 across an incandescent bulb 500 is increased, and a current II starts to flow through the incandescent bulb 500. Then, the triac Tri is kept on until an alternating-current power supply voltage V1 approaches zero volts and the current flowing through the triac Tri reaches a holding current or less.
When the triac Tri within the phase control dimmer 200 is turned on, energy stored in the capacitor CLC flows through the coil LLC, and thus a resonance phenomenon occurs. In a load, such as the incandescent bulb 500 (FIG. 8), that needs a large amount of current, even when oscillation occurs, the triac Tri is not turned off However, in a load, such as the LED load 400 (FIG. 7), whose load is low, the absolute value of a current flowing thorough the triac Tri reaches a holding current (for example, about 10 mA) or less, with the result that the triac Tri may be turned off.
After the triac Tri is turned off by a resonance phenomenon or the like, a trigger voltage is applied again to the triac Tri, and the triac Tri is turned on again within a half period of the same alternating current (in the case of 50 Hz, 10 ms or less of the half period). Here, the timing when the triac Tri is turned on is not stabilized every half period of the alternating current, and thus energy supplied to the LED load 400 is not stabilized, with the result that flickering occurs in the LED load 400.